An air conditioning unit that is running but not cooling — blowing air but not cold air — is one of the most common call-outs we attend. It is also one of the most frustrating, typically happening on the hottest days of the year when you need the system most. Here are the most common causes, in order of likelihood, and what you can do about each one.
Check These Yourself First
1. Mode Setting
This sounds obvious but it is the first thing to check. Remote controls for air conditioning systems have multiple modes — cooling (snowflake symbol), heating (sun symbol), fan only (fan symbol) and auto. If the system has been switched to heating or fan-only mode it will not cool regardless of how high you turn the temperature down. Check the remote and ensure it shows the snowflake cooling symbol with a temperature set below the current room temperature.
2. Dirty Air Filter
A blocked air filter is the single most common cause of poor cooling performance in systems that are otherwise working correctly. The filter sits behind the front panel of the indoor unit and catches dust, hair and particles from the room air. When it becomes heavily loaded it restricts airflow so severely that the unit cannot cool effectively — the system may even ice up on the evaporator coil as a result of the restricted airflow.
Remove the front panel of the indoor unit (it clips off on most systems), slide out the filter, and hold it up to the light. If you cannot see light through it, it needs cleaning. Wash it under a tap, allow it to dry completely, and refit. This takes 10 minutes and fixes the problem immediately in many cases.
3. Outdoor Unit Blocked or Overheating
The outdoor unit needs clear airflow to reject heat effectively. If it has been blocked by overgrown plants, stored items, or if it has been installed somewhere with poor air circulation (inside a cupboard, in a confined space between buildings), it will struggle to cool effectively in hot weather. Check that there is clear space around the outdoor unit — at least 30cm on all sides and ideally 1 metre in front of the discharge grille.
4. Doors and Windows Open
Air conditioning works by cooling the air in a sealed space. If doors and windows are open the system has to work against continuous heat ingress and will struggle to reach the target temperature. Close all windows and external doors when running the system in cooling mode.
Causes That Require a Professional
5. Refrigerant Leak
This is the most common serious cause of air conditioning not cooling. Refrigerant is the fluid that circulates between the indoor and outdoor units, absorbing heat indoors and rejecting it outside. If the system develops a leak, refrigerant level drops and cooling capacity falls — often progressively over weeks or months until the system barely cools at all.
Signs of a refrigerant leak include: gradual loss of cooling over time, ice forming on the indoor unit or pipework, a hissing sound near pipework joints, or the system running continuously without reaching the target temperature. Refrigerant leaks cannot be fixed by topping up alone — the leak must be found and repaired before recharging. Under F-Gas regulations, only a certified engineer can handle refrigerant.
We see refrigerant leaks most commonly at flare joint connections (where pipework connects to the indoor and outdoor units), at joints in extended pipework runs, and occasionally at the compressor valve connections on the outdoor unit.
6. Dirty Evaporator or Condenser Coils
The evaporator coil in the indoor unit and the condenser coil in the outdoor unit are the heat exchange surfaces. Over several years of operation both coils accumulate a layer of dust, mould and debris that insulates the coil surface and reduces heat transfer efficiency. A heavily contaminated coil can reduce cooling capacity by 20-30%.
Coil cleaning is part of our annual service. It requires specialist coil cleaning chemicals and should not be attempted without proper training — incorrect cleaning can damage the coil fins or introduce moisture into electrical components.
7. Compressor Fault
The compressor is the heart of the air conditioning system — it pumps refrigerant around the circuit. A failing compressor produces reduced cooling, often with unusual sounds (rattling, grinding or a laboured startup). Compressor replacement is a significant repair — depending on the system age and the cost of a replacement compressor, it may be more economical to replace the entire system rather than just the compressor.
8. PCB (Circuit Board) Fault
Modern air conditioning systems are heavily electronics-dependent. A fault on the main PCB of the indoor or outdoor unit can cause a wide range of symptoms including failure to cool, erratic operation, or complete shutdown. PCB faults often show as error codes on the indoor unit display — refer to your installation manual for the fault code guide, or call us and describe the code.
9. Expansion Valve Fault
The expansion valve controls the flow of refrigerant into the evaporator. A stuck or faulty expansion valve results in either too much or too little refrigerant flow, both of which reduce cooling performance. This is typically diagnosed by measuring system pressures and temperatures.
When to Call Us
If checking the mode setting, cleaning the filter and clearing the outdoor unit does not resolve the problem, call us. We carry diagnostic equipment to measure system pressures and identify refrigerant leaks, and we stock common repair parts for all major brands.
For emergency call-outs — particularly server rooms and commercial premises where air conditioning failure cannot wait — we offer same-day attendance across Surrey, Sussex and South London. Call 07833 053749. For non-emergency repairs, contact us online and we will arrange a visit within 1-3 working days.